Heat-shrinkable films which undergo shrinkage back to the pre-drawn form when heated at a predetermined temperature have been extensively used for labeling a glass or plastic bottle because of its properties that are suitable for printing various articles to attract consumers' attention and for full-wrapping various containers as well as packaging a bundle of goods.
Heat-shrinkable films made of soft polyvinyl chloride (PVC) have recently become disfavored because they exhibit a limited maximum heat-shrinkage ratio and emit toxic pollutants, e.g., dioxins, on combustion. Oriented polystyrene (OPS) films, on the other hand, have uniform shrinking properties and low specific gravities, and they can be easily removed from PET bottles for recycling, but they have the problem of poor heat-resistance. Therefore, such oriented polystyrene films are unsuitable for a high rate-shrinkage process or high temperature-filling process.
Heat-shrinkable polyester films which have improved shrinking properties and heat-resistance as compared with those of PVC and OPS films have been developed for wrapping a glass bottle. However, the shrinkage rates of such polyester films are generally unacceptably high, which results in non-uniform shrinkage when subjected to a thermal shrinkage process using hot air, and therefore, it necessary to use a steam-heating type shrinking machine to prevent non-uniform shrinkage of the film. In addition, the shrinkage stress of such polyester film is unacceptably high, which often leads to non-uniform shrinkage with consequential distortion, end-bending or rupture, especially when subjected to a secondary thermal shrinkage process, e.g., a sterilization or high temperature-filling process.
In order to solve the above problems, Korean Patent Laid-Open Publication No. 2002-0063158 discloses a copolymer composition comprising terephthalic acid, ethylene glycol, 1,4-cyclohexanedimethanol, and diethylene glycol, and a heat-shrinkable film prepared using the same which has a high softness and a PVC-like shrinking property; and Korean Patent Laid-Open Publication No. 2002-0062838 discloses a heat-shrinkable polyester film comprising a polyester elastomer in an amount of 5 wt % or more.
However, such heat-shrinkable polyester films shows a skirt phenomenon that a film labeled on a container having fixing point such as a square shape container is riding up at the center between the fixed points in the perpendicular to shrinkage direction, or a riding up phenomenon that a film labeled on a round container is wholly riding up in the perpendicular to shrinkage direction, and thus these conventional films cannot be effectively used for the protection of containers or indication of commercial product.
Moreover, conventional polyester films are also known to have shortcomings such as non-uniform heat-shrinkage and crinking during a high temperature shrinking process, which cause the deterioration of the appearance and quality of labeled containers.